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1
Fundamentals and Basic Terminology
LEARNING OBJECTIVES
In this chapter, the reader is introduced to:
• fundamentals and basic terminology of noise control;
• noise-control strategies for new and existing facilities;
• the most effective noise-control solutions;
• the wave equation;
• plane and spherical waves;
• sound intensity;
• units of measurements;
• concepts of sound pressure, intensity and power level;
• frequency analysis and sound spectra;
• adding and subtracting sound levels;
• three kinds of impedance; and
• flow resistance.
1.1 Introduction
The recognition of noise as a source of annoyance began in antiquity, but the relationship, sometimes subtle, that may exist between noise and money seems to be a development of more recent times. For example, the manager of a large wind tunnel once told one of the authors that in the evening he liked to hear, from the back porch of his home, the steady hum of his machine approximately 2 km away, for to him the hum meant money. However, to his neighbours it meant only annoyance and he eventually had to do without his evening pleasure.
The conflicts of interest associated with noise that arise from the staging of rock concerts and motor races, or from the operation of airports, are well known. In such cases, the relationship between noise and money is not at all subtle. Clearly, as noise may be the desired end or an inconsequential by-product of the desired end for one group, and the bane of another, a need for its control exists. Each group can have what it wants only to the extent that noise control is possible.
The recognition of noise as a serious health hazard is a development of modern times. The World Health Organisation (World Health Organisation, 2011) has stated that ‘There is sufficient evidence from large-scale epidemiological studies linking the population’s exposure to environmental noise with adverse health effects. Therefore, environmental noise should be considered not only as a cause of nuisance but also a concern for public health and environmental health.’
With modern industry has come noise-induced deafness; amplified music also takes its toll. While amplified music may give pleasure to many, the excessive noise of much modern industry probably gives pleasure to very few, or none at all. However, the relationship between noise and money still exists and cannot be ignored. If financially compensating people who become deaf is little more expensive than implementing industrial noise control, then the incentive definitely exists to do nothing, and hope that such a decision is not questioned.
A common method of noise control is a barrier or enclosure and in some cases, this may be the only practical solution. However, experience has shown that noise control at the design stage is generally accomplished at about one-tenth of the cost of adding a barrier or an enclosure to an existing installation. At the design stage, the noise producing mechanism may be selected for least noise and again experience suggests that the quieter process often results in a better machine overall. These unexpected advantages then provide the economic incentive for implementation, and noise control becomes an incidental benefit. Unfortunately, in most industries engineers are seldom in the position of being able to make fundamental design changes to noisy equipment. They must often make do with what they are supplied, and learn to apply effective ‘add-on’ noise-control technology. Such ‘add-on’ measures often prove cumbersome in use, and experience has shown that quite often ‘add-on’ controls are quietly sabotaged by employees who experience little immediate benefit and find them an impediment to their work.
In the following text, the chapters have been arranged to follow a natural progression, leading the reader from the basic fundamentals of acoustics through to advanced methods of noise control. However, each chapter has been written to stand alone, so that those with some training in noise control or acoustics can use the text as a ready reference. The emphasis is on sufficient precision of noise-control design to provide effectiveness at minimum cost, and means of anticipating and avoiding possible noise problems in new facilities.
Simplification has been avoided so as not to obscure the...
